1. Summarise the issue (what were doing, what went wrong?)

I was trying to build the darcs-screened repos. I need to enable RTS to 
avoid certain out-of-memory errors. Here's the incantation that's worked in 
the past:

GHCRTS=-K100M cabal -frts -ftest --with-ghc=/home/mark/ghc/bin/ghc install 
-j

Now it results in:

####
setup: Most RTS options are disabled. Link with -rtsopts to enable them.
####

Unfortunately, I have no idea how to "Link with -rtsopts", and the 
diagnostic message doesn't tell me now to do so.

2. What behaviour were you expecting instead?

A clean build.

3. What darcs version are you using? (Try: darcs --exact-version)

I'm trying to build with GHC 7.4.2.

4. What operating system are you running?

Ubuntu 10.04.

The root cause was that I had added "GHCRTS=-K100M" to my .bashrc, 
because I need it for some darcs invocations, and it harmless to the 
rest of the darcs invocations. 

Next time I went to run "cabal install", "cabal" responded to this 
environment variable with this error message:

"setup: Most RTS options are disabled. Link with -rtsopts to enable 
them"

cabal should have either:

1. Explicitly unset or ignored the environment variable if it doesn't 
need it.

or 

2. Noted that it was exiting because of this environment variable, and 
that unsetting the environment variable was another possible solution 
besides relinking. (And it's still not clear what it was suggesting that 
I relink, or how). 

This invocation works for me, by explicitly unsetting the variable for 
this call:

GHCRTS= cabal -frts -ftest --with-ghc=/home/mark/ghc/bin/ghc install 
-j